Control means for electrical apparatus



Oct. 29, 1968 D. GURWICZ CONTROL MEANS FOR ELECTRICAL APPARATUS Filed Aug. 16, 1967 INVENTOR. ISAAC DAVID GURWICZ United States Patent ABSTRACT OF THE DISCLOSURE A control system for a load in which an auxiliary contactor closes before and opens after the main shorting contactor when it is desired to change the supply arrangement to the load and thus prevents arcingat the shorting contactor.

This invention is for improvements in or relating to control means for electrical apparatus and is a continuationin-part of patent application Ser. No. 404,630, filed Oct. 19, 1964, now abandoned.

The invention is concerned with a control system for electrical apparatus comprising a pulse generator circuit adapted to be inserted between the supply and the apparatus to be controlled, and means for adjusting or modulating the mark-space ratio of the pulses and thereby the mean power applied to the motor. For instance, said means may be designed to adjust or modulate the pulse frequency or the pulse width.

In apparatus of the character with which the present invention is concerned it is convenient to provide, in some cases, means whereby the pulse control system can be rendered inoperative and control of the apparatus (e.g. a

. motor or motors) switched, for example, to an alternative control arrangement. For example, it may be desirable when a motor under the control of the pulse control system has been brought to full speed or load, to switch out the pulse control system and connect the motor or the motors directly to the supply through a contactor.

Switching over from pulse control to a normal or other form of control is conveniently effected by means of a shorting contactor. Such a contactor has, heretofore, been subject to heavy arcing which is objectionable since flameproofing of the control gear is required. While previous attempts to suppress arcingof such a contactor have been partially successful, as for example the technique described in US. Patent No. 2,619,524, these attempts have not succeeded in completely suppressing arcing during the making of the contactor and have not treated the problem of arcing suppression during both the making and breaking of the contactor. An object of the present invention is, therefore, to overcome this problem by eliminating arcing at the shorting contactor during periods of both making and breaking.

According to the present invention means are provided to ensure that the main controlled rectifier of the pulse control system is in the conducting condition before the shorting contactor closes, and remains so until the contacts of the shorting contactor, on breaking, have separated sufliciently to prevent arcing.

The invention will be further described by way of example with reference to the accompanying drawing which is a diagram showing a pulse control system with at short circuiting contactor S.C. across a main controlled rectifier SCR an extinguishing circuit for SCR including a second controller rectifier SCR and a capacitor C, and an auxiliary contactor S.C. INT connected, as shown by the dotted line, with the shorting contactor S.C.

As shown in the drawing, a load MO is coupled to a source of power B, indicated by a series of batteries.

The power through the load MO, when a shorting contactor S.C. is open and a main switch MS is closed, is controlled and interrupted by the action of a pulse control circuit consisting of a main silicon controlled rectifier SCR an extinguishing circuit consisting of a second controlled rectifier SCR and a capacitor C, and a variable relaxation oscillator L coupled to SCR lead A and a delay circuit M. The lead A feeds a varying potential into the oscillator L and the delay M. In the normal operation (when S.C. is open), assuming SCR is non-conducting, the potential of lead A rises to full positive from battery B (through diode D and load MO). This starts variable relaxation oscillator L, which, after a delay determined by the variable resistor RF, emits a firing pulse to SCR which turns SCR on. Due to the resultant collapse of potential across SCR lead A can no longer be held positive via diode D As the potential on lead A falls it starts fixed delay M which emits a firing pulse to SCR, which, as explained hereafter, extinguishes SCR As a result, the load MO is subject to a pulsed mean current of fixed pulse length and variable pulse frequency.

The extinguishing circuit for SCR acts as follows. On closure of the main switch MS to start operation, capacitor C charges, via diode D the load MO and through SCR which is fired and turned on immediately by the occurrence of a positive potential through R the resistance of Which is high compared with the output impedance of the delay circuit M so as not to interfere with the normal operation of the delay circuit M. This charges capacitor C fully with +v. on its upper electrode. When SCR is later fired and turned on by the variable oscillator L, as explained previously, capacitor C discharges via SCR inductor L and rectifier D and turns off SCR, by reverse biasing it. Owing to the inductance of L, the charge on C reverses and remains so with the upper electrode being at v. because of D When the delay circuit M refires SCR capacitor C momentarily carries the main current through MO, and reverse biases SCR which thus turns off, SCR remains conductive by current via R until turned off by the next firing of SCR and the cycle recommences under control of oscillator L and delay M.

Diode D acts as a free-wheel shunt across the load M, carrying the inductive current (if any) of M during the off periods of SCR When the oscillator M reaches maximum frequency, it is desired to switch SCR and thus the pulse control system, out of circuit. To accomplish this the coupled shorting contactor S.C. and the auxiliary contactor S.C. INT are closed.

The auxiliary contactor S.C. INT is mechanically arranged to close just before the shorting contactor S.C. closes, and to open shortly after S.C. opens. This can be done, for example, by a simple eccentric rotary cam arrangement. Due to the presence of the diode D point A is held continuously at the supply potential B when S.C. INT is closed, whether or not the silicon controlled rectifier SCR is conducting. This results in the removal of any triggering potential by delay M at the gate of silicon controlled rectifier SCR and the continuous generation of firing impulses from the oscillator L. Silicon controlled rectifier SCR therefore becomes conducting before the shorting contactor S.C. is closed. When the shorting conducting mode when S.C. is closed and throughout any bouncing of S.C. (due to the presence of R which keeps SCR conducting). This firing condition on SCR remains continuous because of the S.C. INT closure, until just after the shorting contactor S.C. opens.

Resistor R ensures that capacitor C is fully charged, in the extinguishing direction, during the period when the shorting contacts S.C. is closed. When the shorting contactor S.C. is opened, the silicon controlled rectifier SCR remains in the conducting mode untilthe opening of the S.C. INT. Breaking of this contactor auxiliar y contactor reactivates the off delay circuit M, and, since C is charged, silicon controlled rectifier SCR off in the normal manner.

The resistor R supplies a firing potential to SCR which ensures that it is in the conducting mode when the equipment is initially switched on.

In the circuit shown in the drawing, if the auxiliary contactor S.C. INT and diode D arrangement were to be omitted, arcing would occur every time the shorting contactor S.C. were closed and opened even momentarily.

This is apparent since, assuming that the main silicon controlled rectifier SCR is initially in the conducting mode, the closing of the shorting contactor 8.0. would immediately transfer the load current of SCR to the shorting contactor 8.0, and would cause SCR to extinguish. If the shorting contactor S.C. bounced on making, then the fully supply voltage would appear across the shorting contactor tips as they broke, causing arcing and possibly welding as the shortening contactor remade. Arcing on breaking of the shorting contactor would be due to a similar reason since the main silicon controlled rectifier SCR would be in the off mode. This objectionable arcing is prevented as described above by ensuring through the auxiliary shorting contactor S.C. INT that the main silicon controlled rectifier SCR is in the conducting mode before the contactor closes, and remains so until the shorting contactor tips on breaking have separated sufficiently to prevent arcing.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by Way of illustration and example only and is not to be taken by way of limitation, the spirit and scope capacitor is turned of thisinvention being limited only by .the terms of the appended claims.

I claim:

1. A control system comprising: a load; means for supplying power to said load, said means including a substantially constant source ofpowemja plurality of controlled rectifiers coupled to said load and said source of power and adapted to interrupt the power supplied to said load in a preselected manner; oscillator means for controlling the conductive condition of at least one of said controlled rectifiers to interrupt said power-supplied; means for short-circuiting said controlled rectifiers for continuously applying said source of power to said load; and interlock means coupled to said source of power and said means for controlling said conductive condition, said interlock means being coupled to said means for shortcircuiting'said controlled rectifiers and adapted to close before and open after the closing and opening, respectively, of said last recited means, and said oscillator means being responsive to said source of power upon the closing of said interlock means to maintain at least one of said controlled rectifiers in a conductive condition upon the closing of and until the opening of said interlock means.

References Cited.

UNITED STATES PATENTS 2,441,789 5/ 1948 Bivens 317l1 2,789,253 4/ 1957 Vang 317-ll 3,237,030 2/1966 Coburn 3l7ll X LEE T. HIX, Primary Examiner. J; D. TRAMMELL, Assistant Examiner. 

